1. Field of the Invention
This invention relates generally to a technique for removing heat from a fuel cell stack and, more particularly, to a technique for removing heat from a fuel cell stack that includes employing a heat pump to increase the temperature of heated coolant from the fuel cell stack to provide a greater temperature difference between the heated coolant and the ambient air to more effectively reject the heat.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode charge gas that includes oxygen, and is typically a flow of forced air from a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product.
The humidity or wetness of the membranes in a fuel cell stack is an important design criteria for effective stack operation. Too much water within the stack acts to prevent the oxygen in the cathode input gas from reaching the catalyst on the cathodes. Too little water within the stack causes the stack membranes to dry out and become more susceptible to cracking and other damage. The more current that the stack generates, the more water is generated as a by-product of the electrochemical process. However, the more air that is forced through the stack by the compressor to provide more current, the more the stack membranes dry out. Typically, the stack has a 110% relative humidity during its most efficient operation. For 110% relative humidity, the exhaust gas is saturated 100%, and also includes a little bit of excess water.
One factor that affects the stack relative humidity is stack temperature. As the stack temperature increases, the stack's ability to hold water in the vapor state also increases making it more difficult to maintain a desired stack relative humidity because more water is required to do so. Another factor that affects stack relative humidity is the stack pressure. As the pressure in the stack increases, the ability of the stack to hold water in the vapor state decreases. Thus, one of the most commonly used techniques to control cathode relative humidity is to control the fuel cell system pressure and temperature.
Fuel cell systems must reject waste heat generated by the fuel cell operation to prevent the stack membranes from drying out and being damaged. Typically, the fuel cell must operate at temperatures near 80° C. to provide the desired performance. A fuel cell system will include a thermal coolant sub-system that removes heat from the stack so that it operates at its desired operating temperature for proper membrane humidity. Heated coolant from the stack is directed to a radiator that reduces the temperature of the coolant so that it can be returned to the stack to remove the stack waste heat. The radiator uses ambient air to reduce the temperature of the coolant, and it is the difference between the temperature of the heated coolant and the ambient temperature that determines how effective the radiator will be at removing heat from the coolant for a particular radiator size.
It is desirable to reduce the size of the radiator as much as possible to conserve space and vehicle weight. However, the smaller the radiator, the less heat the radiator can reject. As discussed above, if the temperature of the fuel cell stack is too high, the membranes within the stack can be damaged. Therefore, there is a limit as to how small the radiator can be reduced relative to the stack requirements so that the proper amount of heat is removed.